The R-T-B based magnet comprising a tetragonal compound R2T14B as the main phase is known to have excellent magnetic properties, and has been considered as a representative permanent magnet with high performances since it was invented in 1982 (Patent Document 1: JPSho59-46008).
Although the R-T-B based magnet has excellent magnetic properties, the trend that the corrosion resistance is low exists due to having the rare earth element that is easily oxidized as the main component.
Therefore, in order to improve corrosion resistance of the R-T-B based sintered magnet, the surface treatment such as coating resins, plating or the like on the surface of the magnet body is usually adopted. On the other hand, by changing addition elements of the magnet body or internal structure, the study on improving the corrosion resistance of the magnet body is performed. Enhancing corrosion resistance of the magnet body is very important for improving reliability of the products after surface treatment. In addition, the simpler surface treatment also can be used than coating resin or plating so as to be advantageous for reduce the product cost.
In the prior art, for example, Patent Document 2 (JP Hei4-330702) has disclosed a technical solution in which the intermetallic compound R—C of rare earth elements between the non-magnetic R-rich phase and carbon is inhibited to be 1.0 mass % or less and corrosion resistance of the magnet is enhanced by reducing the content of carbon in the permanent magnet alloys to 0.04 mass % or less. Further, Patent Document 2 has disclosed a technical solution in which corrosion resistance is improved by setting the concentration of Co in the grain boundary phase to 5 mass % to 12 mass %.